Our Darkest Hour by Brian Rivera

Everything that happens must happen for a reason. Matthew, King of Hades, knows this and must use his brilliance to battle an unseen force to save the world we know. In a land of hidden magic, he battles to preserve life for humanity in a race against time. Meanwhile, Cyrus, King of Poseidon, wrestles with an entirely different threat. Creatures, unnatural and unrelenting, threaten to destroy the balance. They move to destroy modern humanity and the question becomes: Can Matthew and Cyrus out think and out match the enemy before it's too late?